There exist very numerous patent documents illustrating structures for articulated three-dimensional measuring appliances including a plurality of joint axes. Thus, recent measuring appliances have up to six axes.
Reference can be made to documents U.S. Pat. No. 5,402,582, U.S. Pat. No. 5,611,147, U.S. Pat. No. 5,794,356, and U.S. Pat. No. 5,926,782. In general, articulated three-dimensional measuring appliances conventionally comprise a moving assembly rigidly secured to a jointed end of an arm and mounted in the tubular body of the arm to turn about the longitudinal axis of said arm, and a fixed assembly mounted to the other end of the arm, together with connection means for providing an electrical link between the two assemblies.
More recently, it has been found advantageous to improve the conventional structures of articulated measuring appliances so that the operator can cause at least one of the arms to turn through several revolutions about its central axis. However, articulated three-dimensional measuring appliances are conventionally fitted in each arm with an angle coder enabling rotation about the corresponding axis to be measured. Consequently, the desire for a very large range of rotation, or even an infinite range of rotation, requires a solution to the problem of providing a connection to establish the electrical link between the fixed elements and the moving elements.
In this respect, reference can be made to documents U.S. Pat. No. 4,888,877, U.S. Pat. No. 4,593,470, and DE-A-41 40 294.
More recently, arrangements have been developed making use of rotary connectors with circular tracks, using technology that is already well known in couplings for machine members, so as to achieve a range of rotation that can be considered as being infinite. In this respect, reference can be made to document U.S. Pat. No. 5,829,148. That document teaches providing at least some of the joints with a degree of freedom that enables them to be swept over an unlimited arc, with a rotary connector subassembly having multiple contacts for transmitting electrical signals through such joints.
However, none of the solutions set out in the above-mentioned patents gives full satisfaction.
If the range of rotation possible at a joint between two adjacent arms in a three-dimensional measuring appliance is to be limited, it is possible to provide an abutment system associated with a desired maximum range of rotation, e.g. at least 700° to 720°. If the abutment is a rigid mechanical abutment, making contact inevitably leads to a jolt which is transmitted along the articulated structure, which inevitably leads to the appliance losing calibration. Under such circumstances, it is necessary to recalibrate the appliance prior to taking new measurements. In order to avoid such jolting, it is possible to provide an abutment that is more flexible, e.g. in the form of a fusible abutment. However, under such circumstances, on coming into abutment, if the operator applies a certain amount of torque to the corresponding arm, then the abutment ruptures, thereby avoiding generating excessive jolting. However, in spite of the improved security that is thus obtained, it is not possible to prevent the electrical connection wires becoming twisted and knotted or breaking. This means that the various abutment systems that might be envisaged are considered as being not being very satisfactory.
To avoid the above-mentioned drawbacks inherent to rigid or flexible abutments, the only proposals that have been made relate to rotary connectors with circular tracks and wipers. In particular, above-mentioned document U.S. Pat. No. 5,829,148 discloses a rotary connector system having a cylindrical drum carrying circular tracks which are contacted by associated fixed wipers. However, that type of arrangement is not completely satisfactory concerning the quality of the electrical contact between the wipers and the tracks. Consequently, it is not possible to avoid generating interference which disturbs the measurement signal. Furthermore, the need to exert rotary torque of non-negligible level on the arm concerned also runs the risk of interfering with measurement accuracy: even a small amount of torque applied to a jointed arm of a multiple-arm measuring appliance can lead to an error of a few microns in the terminal sensor, thereby falsifying measurement to a greater or lesser extent.